The invention disclosed and claimed herein generally pertains to a method and apparatus for producing comfort noise in a system for transmitting information, wherein transmissions are intermittently discontinued. More particularly, the invention pertains to a system for transmitting human speech, wherein transmissions are discontinued during speech pauses. Even more particularly, the invention pertains to a communication system which is disposed to transmit information through an air interface.
There is increasing interest in providing mobile phones and other small electronic devices with the capability to connect and communicate wirelessly, over short ranges, with one another. Such capability would eliminate or substantially reduce the need for cables between devices such as phones, PC cards, headsets and laptops computers. Moreover, a number of such devices could thereby be joined together, very readily, to form small networks. As an example of this interest, the assignee herein, a major supplier of mobile telecommunication equipment and systems, has initiated a program known as the Bluetooth air interface to develop wireless communication capability of the above type.
In interconnecting small devices over a short range air interface, it is very important to restrain costs, and also to reduce the overall interference level on the interface, in order to improve RF spectrum efficiency. Accordingly, in an air interface such as the Bluetooth interface, a transmitter disposed to receive speech is provided with a discontinuous transmission (DTX) capability, which causes the transmitter to be switched off during speech pauses. Such capability reduces costs, by minimizing transmitter power requirements and also reduces signal interference level. The benefits of DTX may be particularly significant in connection with telephone transmissions, in view of the fact that during a normal phone conversation, the participants alternate so that, on average, each direction of transmission is utilized only about 50% of the time.
Notwithstanding the benefits and advantages of discontinuous transmission, background noise, which is transmitted together with speech in a system of the above type, disappears during silent or speech absent periods when the transmitter is switched off. This results in modulation of the background noise. More particularly, since switching of the transmitter between its transmit and transmit discontinue modes can be quite rapid, a very annoying noise effect can be produced at the receiver.
A principal technique of the prior art, to avoid such annoying noise, has been to model the background noise by speech encoder model parameters which are updated at low rate intervals. The model parameters are then used to generate comfort noise signal on the receiver side of the communication system, during periods when the transmitter is turned off. Such a scheme is generally suitable when both a model-based speech coder and a high performance DSP (Data Processor?) are available, in order to generate the required comfort noise at the receiver side. However, in short range air interface systems of the type described above, such as Bluetooth, these assumptions are not applicable. Neither the model parameters for the background noise, nor a DSP of sufficient computational capability, are typically available. Moreover, the voice coder in Bluetooth is a waveform coder with a one-bit quantization scheme, called the continuously variable slope delta modulation (CVSD). The 64 kb/s CVSD is a non-linear waveform coder with no model information available, apart from instantaneous waveform amplitude and slope information. These parameters are not amenable for modeling background noise, due to their rapidly varying nature.
In the invention, it has been recognized that background noise perception is based primarily on power level or amplitude of the noise information. This is true because the amplitude modulation (between silence and noise-containing speech components) of the background noise is the principal cause of the annoying noise which occurs during DTX switching of the transmitter. Accordingly, comfort noise generated at the receiver, at an amplitude which is substantially equal to the transmitter side amplitude, enhances speech quality and significantly diminishes the annoying effects described above. In accordance with the invention disclosed herein, adequate comfort noise may be provided, simply by judicious adjustment of a comfort noise generator located at the receiver side of the transmission interface. Thus, the invention provides a comparatively simply technique based on auditory perception of background noise, relative to the speech signal.
In one embodiment, the invention is generally directed to apparatus for providing comfort noise in a communication system used in connection with an audio voice signal comprising interspersed speech present periods and silent periods, wherein the communication system comprises a transmitter and receiver at opposing sides of a transmission path or interface, and the transmitter is provided with the aforesaid DTX capability. Thus, the transmitter is disposed to transmit the audio signal to the receiver during speech present periods, and to discontinue transmissions during silent periods. The apparatus comprises a comfort noise generator at the receiver side of the interface for producing comfort noise of adjustable amplitude. The apparatus further comprises a first subsystem for detecting silent periods at the transmitter side, and for providing notice to the receiver when the transmitter discontinues transmission of the audio signal. A second subsystem is provided for transmitting a succession of amplitude parameters through the interface to selectively adjust the amplitude of the generator, in corresponding relationship with a noise level detected at the transmitter.
In a preferred embodiment of the invention, the first subsystem includes a voice activity detector incorporated into the transmitter for producing speech present and speech absent flags in response to speech present and silent periods, respectively, occurring in the audio signal. In a useful embodiment, the speech present and speech absent flags respectively comprise first and second digital bits, which are transmitted across the interface to the receiver. In an alternative embodiment, wherein voice information is transmitted across the interface in digital packets, the occurrence of a particular silent period which causes transmission to discontinue, is indicated to the receiver by preventing transmission of a voice information packet which corresponds to the particular silent period.
Usefully, the second subsystem comprises a device for providing periodic estimates of the noise level at the transmitter, and further comprises means for transmitting amplitude adjustment parameters, which respectively represent the noise level estimates, across the interface to the comfort noise generator.